Why are so many fossil land- dwelling mammals found in the western united states, but so few in the eastern part of the country?

It's because the flow of the stream.
The flow of the stream flow towad the north-western direction, so all parts of the fossils that died million years ago ended up to be formed on the western region.
To this date, the western united states provide the majority set of fossils that ever found in united states.

To have or to find and keep